The present invention relates to x-ray mirrors, particularly to multilayer mirrors, and more particularly to a process for fabricating beryllium-based multilayer x-ray mirrors useful in the wavelength region greater than the beryllium K-edge (11.1 n).
In the past few years, much attention has been directed to the development of x-ray optical applications, such as x-ray microscopy, astronomy, and extreme ultraviolet (EUV) lithography (EUVL), also known as soft x-ray projection lithography (SXPL). The use of shorter wavelength radiation allows thinner lines to be etched in the photoresist, used to delineate features during integrated circuit manufacture. Reducing the line width (and hence the design rules) will make greater circuit densities and further miniaturization possible. EUVL is considered as a possible route to very-large-scale integrated circuit manufacturing with feature sizes of 0.1 xcexcm and smaller. The key item, without which the EUVL technology would be impossible, is high-reflectance multilayer mirrors for wavelengths near 10-15 nm. Some proposed systems use as many as eight normal-incidence reflections between the source and the resist-coated wafer, and for this reason, high mirror reflectance is essential. An EUVL system requires normal incidence mirror maintaining reflectance greater than 60% for x-rays in the range of 6.0-14.0 nm (60-140 xc3x85).
Until now, molybdenum/silicon (Mo/Si) mirrors have received the most attention for EUVL and they have emerged as the de-facto standard, because this combination shows high theoretical reflectance (xcx9c76%) with 13.0 nm (130 xc3x85) illumination. Intense activity has been devoted to efforts to achieve a reflectance close to this theoretical value and to demonstrate the thermal, radiation, and time stability of the multilayers. Measured reflectances of 63-65% are now achieved routinely with the Mo/Si multilayer mirror system. Normal incidence reflectivity as high as 66% at 13.4 nm (134 xc3x85) has been achieved using Mo/Si multilayer coatings, and this material combination is generally considered to be optimal for x-ray wavelengths above the Si L-absorption edge at 12.4 nm (124 xc3x85).
While Mo/Si has been the de-facto standard for EUVL mirrors because it is relatively easy to deposit with sufficient peak reflectivity (xcx9c64%), this material system is limited to wavelengths longer than the silicon LII-III edge at 12.6 nm (126 xc3x85). Researchers have designed their mirrors for the  greater than 130 xc3x85 (13.0 nm) wavelength region; however, some subsystems (e.g. photoresists) may require shorter wavelength radiation. The x-ray penetration depth is larger at shorter wavelengths, which can allow the use of single-layer resists in lithography.
As indicated above, there is a need for reflectivity mirrors below 130 xc3x85 (13.0 nm) in SXPL systems for practical industrial processes. Just below the silicon LII-III edge, beryllium-based multilayers show the highest theoretical peak reflectivity, but these multilayers are limited to wavelengths longer than 111 xc3x85, 11.1 nm (Be K-edge). However, this 111-130 xc3x85 wavelength region is potentially useful and could allow a xcx9c20 xc3x85 shift in the design wavelength for SXPL systems. The corresponding increase in photon energy should make current photoresist materials compatible with the photolithographic process.
There are other motivations for developing multilayer x-ray optics for operation at wavelengths shorter than 12.4 nm, where use of Mo/Si is no longer feasible. For example, in both astronomy and microscopy, imaging at shorter wavelengths can provide an improvement in optical resolution. In addition, the greater penetration depth at shorter wavelengths should allow the use of thicker specimens in microscopy.
The best neutron supermirrors currently in production are made of alternating layers of nickel (Ni) and titanium (Ti), with carbon (C) added to the Ni to eliminate heteroepitaxial growth. Reflectivities of 75-95% in the 0C-20C range are currently obtained from the Ni/Cxe2x80x94Ti system. Also, titanium, beryllium (Tixe2x80x94Be) multilayer systems have been considered as an alternative to the Ni/Cxe2x80x94Ti system. The Tixe2x80x94Be system makes for bilayer structures which have excellent neutron contrast, a necessary requirement of super-mirror devices. Such a Tixe2x80x94Be multilayer system is described in xe2x80x9cNeutron Reflectivity Measurements of Titanium-Beryllium Multilayers,xe2x80x9d A. E. Munter et al., University of Illinois at Urbana-Champaign, Department of Nuclear Engineering, dated Jun. 23, 1995.
Since beryllium dust is considered toxic, very little work has been done in the beryllium-based multilayer area. Also, oxygen uptake has limited x-ray performance of previous work. However, recent efforts in the development of the present invention have overcome any technical problems and any industrial safety/industrial hygiene problems and developed a safe method to produce very pure beryllium films, with interfacial roughness comparable to or better than the Mo/Si systems. The pure beryllium films are produced by sputter deposition and the multilayer x-ray mirrors or optical structures are formed by a process which includes alternating sputter deposition of beryllium and a metal, typically from the fifth row of the periodic table. The present invention includes not only the method of sputtering the materials but also the industrial hygiene controls for safe handling of beryllium including confining of dust and fumes, and filtering to protect the environment. The beryllium-base multilayer structures, such as x-ray optics, of the present invention are particularly applicable for use in soft x-ray and extreme-ultraviolet projection lithography.
It is an object of the present invention to provide beryllium-based multilayer structure for x-ray mirrors.
A further object of the invention is to provide a multilayer mirror having alternating layers of high-Z material and low-Z material.
A further object of the invention is to provide a process for fabricating beryllium-based multilayer x-ray mirrors.
A further object of the invention is to provide beryllium-based multilayer x-ray mirrors useful in the wavelength region greater than the beryllium K-edge (111 xc3x85).
Another object of the invention is to provide a process which includes alternating sputter deposition of beryllium and a metal, typically from the fifth row of the periodic table.
Another object of the invention is to provide a process for sputter deposition of beryllium-based multilayers which includes industrial hygiene controls for safe handling of beryllium.
Another object of the invention is to provide beryllium-based multilayer mirrors for soft x-ray and extreme-ultraviolet projection lithography.
Another object of the invention is to provide a process for fabricating beryllium-based multilayer mirrors with adequate reflectivity (xcx9c60%) for projection lithography.
Other objects and advantages of the invention will become apparent from the following description. The present invention involves beryllium-based multilayer structures which are particularly applicable for x-ray mirrors and a process for fabricating such multilayer structures. The beryllium-based multilayer x-ray mirrors are useful in the wavelength region greater than the beryllium K-edge (111 xc3x85 or 11.1 nm) and are thus useful for example, in soft x-ray projection lithography (SXPL), or extreme-ultraviolet lithography (EUVL) mirrors for soft x-ray synchrotron beamlines. The process includes alternating sputter deposition of a low atomic number (low Z) metal (beryllium) and a high atomic number (high Z) metal (typically from the fifth row of the periodic table), such as niobium, molybdenum, ruthenium, and rhodium. This invention includes not only the method of sputtering the materials but the industrial hygiene controls for safe handling of beryllium, since beryllium dust is considered toxic. The beryllium-based multilayer mirrors made in accordance with the present invention have a reflectivity (xcx9c65%) which is adequate for projection lithography, since SXPL and EUVL require mirrors of a reflectivity  greater than 60% for x-rays in the range of 60-140 xc3x85 (6.0-14.0 nm).